Open questions in earthquake physics
and
the contribution of array seismology
J.-P. Ampuero
Caltech Seismolab
Acknowledgements: Lingsen Meng (now at UC Berkeley)
Overview
• Earthquake physics: how earthquakes start,
propagate and stop? + broader impact
• More specific open questions
• Limitations of source inversion
• Contribution of array seismology
General goals
of earthquake dynamics research
• How earthquakes start, propagate and stop?
• Relation between rupture patterns and mechanical properties of
the fault zone at a variety of scales: friction law, state of stress, offfault inelastic deformation, geometrical roughness
• Fundamental quest: bridge the gap between first principles and
observations at laboratory and natural scales
• Practical impact: physics-based earthquake hazard assessment,
time-dependent hazard, earthquake predictability, situational
awareness, earthquake early warning
Some open questions
in earthquake seismology
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What controls rupture speed?
What controls rise time?
What controls the location of high and low
frequency slip?
How large is the spatial variability of these
source parameters?
How do frictional properties and stress vary
along active faults?
How deep can ruptures propagate?
When and how far can rupture
jump/branch to other fault segments?
How is rupture affected by the subduction
wedge, the fault geometry, the presence of
a low velocity fault zone?
Can slip nucleate repeatedly during a single
earthquake?
from Rippeger et al (2007)
Gabriel et al (2012)
Rupture complexity:
multiple rupture fronts
based on source
inversion by Lee et al
(2011)
Slip rate
Rupture front splitting
Repeated front
Gabriel et al (2011)
Reverse front
Nielsen et al (2010)
Intrinsic limitations of
source inversion
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Source inversion = infer the spacetime distribution of slip from
seismological + geodetic + field +
tsunami + remote sensing data
Only seismological data constrains the
time-dependency of the source
Poor knowledge of the crust structure
at small scales  only low frequencies
(<<1Hz) are exploited
Resulting slip models are notoriously
heterogeneous (spatial variability)
However, the inverse problem is
intrinsically ill-posed  limited
spatial resolution (>10km)
Can we distinguish real source
complexity from inversion artifacts?
What can we trust?
from Martin Mai and SIV project
Earthquake source imaging by
back-projection of teleseismic array data
Introduced by Ishii, Shearer et al (2005)
Principle:
1. Identify coherent wave arrivals across a
dense tele-seismic array
2. Use their differential arrival times to infer
source locations
3. Repeat as the earthquake unfolds, in order
to track the rupture
Seismic
array
Source
region
Seismic rays
(Hutko, 2009)
High-resolution is obtained by exploiting high-frequency waves (~1Hz)
Earthquake source imaging by
back-projection of teleseismic array data
Introduced by Ishii, Shearer et al (2005)
Principle:
1. Identify coherent wave arrivals across a
dense tele-seismic array
2. Use their differential arrival times to infer
source locations
3. Repeat as the earthquake unfolds, in order
to track the rupture
(Hutko, 2009)
High-resolution is obtained by exploiting high-frequency waves (~1Hz)
Earthquake source imaging by
back-projection of teleseismic array data
Advantages of back-projection compared to source inversion:
• High frequency teleseismic data (1 Hz)
• Less affected by uncertainties in velocity model
• Minimal assumptions about fault geometry
• No assumptions on rupture kinematics and size
Challenges and how we addressed them:
• Multiple sources  MUSIC method
• Non-stationary signals  multitaper method
• Swimming artifacts  reference window method
• Limited coherency (see later talks by C. Langston and L. Meng)
Tohoku earthquake
Details of the rupture process
Sketch: position of the rupture front at
regular times
High-frequency radiation is deep
A possible interpretation, :
Deep brittle asperities surrounded by creep
+ Stress concentrations at the edge of past
earthquakes
Similar concept emerged from slow slip and
tremor observations elsewhere
Ito et al (2007)
Ito et al (2007)
Dynamic modeling
Yingdi Luo, earthquake cycle simulations
Percy Galvez
2012 M8.6 Indian
Ocean earthquake
India-Australia
diffuse deformation zone,
an emerging plate boundary
DeMets et al, 2010
Largest strike-slip earthquake ever.
Not really an intra-plate event.
Europe
Japan
Time (s)
As seen from Europe
As seen from Japan
②-NE
④
①
②-SW
③
B
C
A
Rupture branching
despite compressive
dynamic stresses
 low pressuresensitivity of fault
strength
Theoretical expectations confirmed by dynamic rupture simulations
Rupture branching despite compressive dynamic stresses
 low pressure-sensitivity of fault strength
Perspectives
• Contributions of array seismology:
– Provides observational constraints on rupture history with minimal
assumptions
– Breaks the high-frequency (1Hz) barrier in source imaging
– Reveals unexpected rupture patterns
– Provides a fast estimate of rupture size and areas of high-frequency
radiation for situational awareness (ShakeMap in poorly
instrumented regions, or when local system fails)
• Challenges:
– Broadband integration of HF array imaging and LF source inversion
– Optimal data fusion: how to merge information from multiple arrays?
– Quantify and mitigate uncertainties on location, timing and amplitude
of high-frequency sub-sources
– Can we push to higher frequencies > 1Hz? Can we beat scattering?
smaller magnitude earthquakes